It is known to construct a craft, typically a ship, having a so called wingsail in which the sail is formed as a more or less rigid aerofoil section as in an aircraft wing. In such arrangements it is usual to mount the sail on the vessel about an approximately vertical pivot axis. This pivot axis normally comprises a single lower bearing about which the wingsail is rotated in order to adjust the trim angle of the sail to the wind.
In both servomechanically rotated and tail trimmed wingsails, it is desirable to minimize the trimming work needed. For this reason the axis of the trim bearing is usually arranged to pass through the narrow zone in which the center of pressure of the sail is found in the normal working range. However, for most cambered and high thrust sections this zone will lie approximately 35% back from the leading edge, while even in symmetrical sections it will be some 25% back from the leading edge. Its vertical position is approximately at the half height for wingsails of rectilinear design, while simple geometrical constructions are commonly used to locate the zone vertically for wingsails of tapered or elliptical shape.
The strongest part of the wingsail, most suitable for mating with the vertical bearing assembly, is usually a Dbox leading edge section, which may occupy only the leading 20% perhaps of the total wingsail chord.
In the case of a wingsail having two elements, it is normally assumed that the leading element is the principle strength member.
Many wingsail propulsion sets need to be carefully balanced about their vertical or near vertical axes of rotation to enable optimum control of angle of attack to be achieved, and the invention is in part concerned with wingsail balancing.
In complex wingsails, composed of two or more elements hinged or otherwise connected, three distinct types of conformation may be obtained:
These are:
Symmetrical and all-in-line, PA1 Cambered for port tack sailing, and PA1 Cambered for starboard tack sailing.
Such a sailset may be balanced about a vertical or near vertical axis by a mass mounted on a boom and pivoted to the leading edge of the leading sailset section.
The actual construction of a wingsail or the various elements of a multiple element wingsail is important, in that weight and strength are of primary importance. The invention is therefore also concerned with the actual construction of a wingsail and of the elements which make up a multiple element wingsail.